Phased array ultrasonic imaging systems have been used to produce real-time images of internal portions of the human body. Such imaging systems include a multiple channel transmit beamformer and a multiple channel receive beamformer either coupled to a single array of ultrasonic transducers using a transmit/receive switch, or coupled separately to a transmit transducer array and a receive transducer array. The transmit beamformer generates timed electrical pulses and applies them to the individual transducer elements in a predetermined timing sequence. The transducers respond to the electrical pulses and emit corresponding pressure waves, which are phased to form a transmit beam that propagates in a predetermined direction from the transducer array.
As the transmit beam passes through the body, a portion of the acoustic energy is scattered back toward the transducer array from tissue structures having different acoustic characteristics. An array of receive transducers (which may be the same as the transmit array) converts the pressure pulses into the corresponding electrical pulses. Due to different distances, the ultrasonic energy scattered from a tissue structure, arrives back at the individual transducers at different times. Each transducer produces an electrical signal that is amplified and provided to one processing channel of the receive beamformer. The receive beamformer has a plurality of processing channels with compensating delay elements connected to a summing element. The system selects a delay value for each channel to collect echoes scattered from a selected point. Consequently, when the delayed signals are summed, a strong signal is produced from signals corresponding to the selected point, but signals arriving from other points, corresponding to different times, have random phase relationships and thus destructively interfere. The relative delays of the compensating delay elements control the orientation of the receive beam with respect to the transducer array. Thus, the receive beamformer can steer dynamically the receive beam to have a desired direction and can focus the beam to a desired depth.
To collect imaging data, the transmit beamformer directs the transducer array to emit ultrasound beams along multiple transmit scan lines distributed over a desired scan pattern. For each transmit beam, the receive transducer array connected to the receive beamformer synthesizes one or several receive beams having selected orientations. The transmit and receive beams form a round-trip beam (i.e., "center of mass" beam) that is generated over a predetermined angular spacing to create a wedge-shaped acoustic image or is generated over a predetermined linear spacing to create a parallelogram-shaped acoustic image.
Most of the medical ultrasound imaging systems today employ a one-dimensional transducer array to form a two-dimensional image slice through a volume of interest. Increasingly, however, medical practitioners prefer three-dimensional images. To acquire three-dimensional imaging data, the ultrasound system can either use a one-dimensional transducer array that is mechanically moved in a second dimension or can use a two-dimensional transducer array. While the mechanical scanning method may provide good images, the method requires several minutes to obtain a three-dimensional data set. The organ of interest may, however, move during the data acquisition. Therefore, the use of a two-dimensional transducer array is preferred for some purposes.
The two-dimensional array (or even a one and half-dimensional transducer array used for elevation aperture control) may have several hundred to several thousand transducer elements. A basic problem with these large arrays is how to connect them to the receive beamformer, which has a limited number of signal processing channels, such as the 128 channel systems prevalent today. Several solutions were suggested.
One technique employs analog multiplexers that select groups of a reduced number of transducer elements to be connected to the beamformer. The selected group of elements is then electronically updated for each acoustic line. However, in this technique, only a small acoustic aperture is active at any time.
Another technique uses several sub-array receive processors, each connected to a group of receive elements. The sub-array processors provide their outputs to a conventional beamformer. The processors may include analog phase shifters to effectively increase the number of receive elements connectable to available beamformer channels. The use of phase shifters, however, limits the amount of effective delay available within a sub-array processor and thereby limits the number of elements within the sub-array.
Alternatively, one may try to build a beamformer with several hundred or even thousand signal processing channels and connect each channel to one transducer element. Presumably, such arrangement would be very expensive and impractical. Furthermore, this arrangement would require an impractical number of wires within the transducer cable, connecting the transducer handle containing the transducer array and the electronic box with the beamformer.
To pulse the transducer elements, a conventional ultrasound system typically uses a pulse generator for triggering the transmit elements. The pulse generator uses a synchronous counter for counting clock cycles until a desired delay value is reached, and also for counting clock cycles to give the desired pulse width and number of pulses. Since typically one synchronous counter is connected to one transducer element, a system connected to such large two-dimensional transducer array would need hundreds or thousand synchronous counters to provide transmit pulses; this would require a large amount of power and space. Furthermore, the conventional ultrasound system connected to a very large number of transducer elements would require a very large number of receive beamformer channels with delay lines having large delays and fine delay resolution.
In general, there is a need for an ultrasound imaging system that uses a large transducer array for providing two-dimensional or three-dimensional images. The system would need to be practical in size, cost and complexity and sufficiently fast to acquire images of moving organs.